In 1982, UOP first described the preparation of a family of molecular sieves, called aluminophosphates (AIPOs) (Wilson, S T, et al. J. Am Chem. Soc. 1982, 104, 1146). The composition of the microporous crystal structure of these materials consists of Al and P tetrahedra that share vertices via O atoms. Silicoaluminophosphates (SAPOs) are a particular case of AIPOs, where some of the atoms of the structure are partially substituted by silicon atoms (Chen, J S et al. J. Phys. Chem., 1994, 98, 10216). This substitution can occur via two different mechanisms: a) replacement of a P atom by a Si atom, generating a negative charge in the structure (isolated silicon), or b) replacement of one atom of Al and one atom P by two Si atoms, giving rise to the formation of silicon-rich domains (silicon islands). Only when the silicon is in isolation, SAPOs exhibit excellent cation exchange capacity, allowing the presence of different active species suitable for various catalytic applications. Possibly the most common SAPOs are in protonated form. The protons associated with the replacement of the Si structure gives these materials high acidity, which can be applied as acid catalysts in catalytic processes, such as synthesis of methanol to olefins (S W Kaiser, U.S. Pat. No. 4,499,327; 1985).
Thus, the distribution of silicon in the walls of the SAPOs is arguably the most important factor for controlling the acidity of these materials. The organic structure-directing agent (OSDA) used in the preparation of SAPOs not only influences the crystallization of a given structure, but also the positioning and coordination of the silicon atoms in the crystal structure of the molecular sieve.
Silicoaluminophosphate SAPO-18 is a three-dimensional small-pore molecular sieve (eight-atom channel openings with pore diameters of approximately 3.8 Å) with large cavities in its interior. As observed in the literature, the conventional preparation of SAPO-18 was carried out using the organic molecule N,N-diisopropylethylamine as OSDA (Chen et al Catal Lett, 1994, 28, 241; Chen et al. J. Phys Chem, 1994, 98, 10216; Hunger et al, Catal Lett, 2001, 74, 61; Wragg et al J. Catal, 201 1, 397). This SAPO-18 synthesis procedure makes it possible to obtain silicoaluminophosphate with mixtures of isolated silicon and silicon islands. The presence of silicon islands confers lower Brönsted acidity to said SAPO-18s (Chen et al J. Phys Chem, 1994, 98, 10216; Hunger et al, Catal Lett, 2001, 74, 61; Wragg et al. J. Catal., 201 1, 397). Recently, the synthesis of SAPO-18 using tetraethylammonium as OSDA has been disclosed, but this synthesis method also shows large silicon environments forming islands (Fan et al., J. Mater. Chem., 2012, 22, 6568), and therefore, lower Brönsted acidity.
Other cations, different to the protons, may also be introduced in the SAPOs. Conventionally, the introduction of the metal cation species in SAPOs (Me-SAPO) is carried out via post-synthetic cation exchange procedures. However, said post-synthetic procedures require numerous intermediate stages to obtain the Me-SAPO: hydrothermal synthesis of SAPO, calcination, transformation into the ammonium form (if required), cation exchange of the metal and, finally, calcination to obtain the final Me-SAPO. All of these intermediate steps result in an increase in the cost of the synthesis of the corresponding Me-SAPO.
Furthermore, when introducing a cationic metal in extra-network positions in a SAPO, it is very important that the silicon species be isolated in the crystal lattice, because they will generate the negative charges that will make it possible to compensate and stabilise the positive charges of the cationic metals.
In recent years, the preparation of metal-substituted molecular sieves, and particularly, molecular sieves with a small pore size and large cavities in the substituted cationic copper structure have received much attention because of their high activity and stability for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia or hydrocarbons in the presence of oxygen (I. Bull, et al., U.S. Pat. No. 0,226,545, 2008). In this regard, the formation of NOx during combustion of fossil fuels, especially in transport is one of the great current environmental challenges.
Recently, the introduction of Cu cationic species in the SAPO-18 materials by post-synthetic cation exchange has been described (Li et al WO2008/1 18434; Ye et al. Appl Catal A 2012, 427, 34; Ye et al., CN 102 626 653). However, the synthesis of the molecular sieve Cu-SAPO-18 requires a considerable number of steps to finally obtain the catalyst: hydrothermal synthesis of SAPO-18, removal by thermal treatment of organic matter confined in the interior of the pores/cavities during synthesis, prior cation exchange with ammonium cations, cation exchange with copper and, finally, calcination to obtain the Cu-SAPO-18. This material shows good catalytic activity for NOx SCR, but less hydrothermal stability than other catalysts, such as Cu-SSZ-13 or Cu-SAPO-34 (Ye et al. Appl. Catal. A, 2012, 427, 34). The fact that the synthesis of the original SAPO-18 is carried out using the procedure described in the reference (Chen et al. J. Phys. Chem. 1994, 98, 10216), where part of the silicon is forming silicon islands, prevents good stabilisation of extra-framework cations and, therefore, the corresponding Cu-SAPO-18 is less stable.